(1) Field of the Invention
The present invention relates to a black surface-treated steel sheet and a process for the preparation thereof. More particularly, the present invention relates to a black surface-treated steel sheet used for a member to be decorated, and a process for the preparation thereof.
(2) Description of the Related Art
Development of a high-performance surface-treated steel sheet at a low cost is a consistent requirement in the fields of anti-corrosive steel sheets for automobiles, household steel sheets, furniture and construction materials, and the requirement standards have become more severe, and further, a reduction of the cost and an enhancement of the quality are needed. The steel makers have met these users, requirements by developing new techniques and new products, and in the field of products previously prepared by processing a surface-treated steel sheet and pre-treating and coating the steel sheet, serious attempts has been made to introduce a pre-coated steel sheet to this process, to provide high-quality products at a low cost and allow the pre-treating and coating steps by the user to be omitted. Accordingly, a pre-coated steel sheet obtained by coating a steel sheet with a paint has been used, but the requirement for steel sheets colored with an inorganic system is increasing in view of the appearance, weldability, and a prevention of scratches formed during handling.
Black is the most widely-demanded color, and further, there is a demanded for a fingerprint-proof property and a mass productivity, chemical resistance, and corrosion resistance.
Stainless steel sheets, steel sheets, and copper sheets are generally used in the conventional blackening treatment method, but in view of the cost and corrosion resistance, a zinc-plated steel sheet is preferable, to meet the above-mentioned requirements. Accordingly, the conventional blackening techniques using this zinc-deposited steel sheet will be described.
The following techniques are known as methods of blackening zinc-plated or zinc alloy-plated steel sheets by cathodic electrolysis.
Japanese Unexamined Patent Publication No. 60-190588 discloses a method in which the cathodic electrolysis is carried out at 1 to 50 A/dm.sup.2 for 0.5 to 30 seconds in an aqueous solution of an alkali metal salt of sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid or carbonic acid, a telluric acid or organic acid salt, or an alkali metal or ammonium salt of thiocyanic acid, thiosulfuric acid or hypophosphorous acid, which contains CO.sup.2+ or Ni.sup.2+ and is maintained at a pH of 2 to 11.
This method, however, is not fully satisfactory and involves many problems. During tracing experiments of the method of Japanese Unexamined Patent Publication No. 60-190588, conducted by the present inventors, it was found that blackening is possible in the ideal state of using a stationally bath in a laboratory, but the method cannot be applied to the high-speed treatment of a broad hoop, as intended in the present invention, because an uneven appearance is caused.
Japanese Unexamined Patent Publication No. 61-143594 discloses a method for the production of a black steel sheet, which comprises subjecting a steel sheet, plated with an alloy of an element of the iron group and zinc, to anodic electrolysis in an aqueous solution containing an anion such as NO.sub.3.sup.- or SO.sub.4.sup.2-. It is taught in this patent publication that, in view of the corrosion resistance and the like, a structure formed by applying a chromate coating or transparent organic resin on the obtained black coating is preferred.
Japanese Unexamined Patent Publication No. 58-151491 discloses a blackening method in which electroplating is carried out in a zinc-plating solution containing Co, Ni or Mo, anodic electrolysis is carried out in an aqueous solution containing NH.sub.4.sup.+ to form an inorganic black coating and, if necessary, the black coating is covered with a silicate coating.
The anodic oxidation method is a technique in which a flackening can be carried out only on a specific plating such as Zn-Ni alloy, compared with cathodic method. Further, the anodic oxidation method has insufficient corrosion resistance since the plated coating is remelted and its production cost is expensive.
Japanese Unexamined Patent Publication No. 60-200996 discloses a method in which a black zinc alloy-plated coating containing at least 15% of Ni is formed by cathodic electrolysis in an Ni/Zn alloy-plating solution.
These methods, however, are unsatisfactory in that there are problems with the production and the products fail to meet market requirements. For example, the method in which the plating is re-dissolved is disadvantageous from the viewpoint of cost, and since a waste water treatment is necessary, the plating is limited to alloy-plating. This problem is solved by the cathodic treatment, but unevenness easily occurs. Furthermore, when a broad hoop is treated at a high speed, various appearance defects such as color unevenness are observed and the yield is reduced.
Also with respect to quality, the conventional techniques have the following problems.
As pointed out hereinbefore, a surface-treated steel sheet blackened with an inorganic system is in practical use and the demand for this steel sheet has increased. Nevertheless, the following performance improvements are required:
(1) Since the steel sheet is used not only as an interior sheet but also as a part close to an exterior sheet, a stable and uniform hue and a gloss-controlled high-grade black appearance are necessary.
(2) In view of the demand in the field of electronic appliances, a higher electric conductivity is required.
(3) In the field of office machines utilizing light, a further reduction of reflectance is needed.
According to the conventional techniques, to improve the corrosion resistance and other characteristics, an insulative clear organic resin or silicate coating having a thickness of 1 to 3 .mu. is formed as the topcoat, and accordingly, although a satisfactory electric conductivity is maintained upon application of a certain voltage, as at a welding step, the electric conductivity is unsatisfactory in electronic appliances using a low voltage.
As the means for attaining a low reflectance, a clear coating formed by using a delustering silica has been proposed, but a proper balance must be maintained between the thickness of the coating and the size of silica particles, and a gloss and a scratch resistance are not compatible factors. Particularly in the case of copying machines, a reduction of the reflectance is extremely desirably because a high reflectance reduces the copying precision.
In connection with the productivity, as pointed out hereinbefore, in the conventional methods, to blacken a large quantity of a broad hoop, the line speed is reduced and the frequency of exchange of the plating solution is increased to stabilize and uniformalize the hue, even if the composition of the plating, the flow rate of the plating solution, and the composition of the plating solution are changed.